1. Field of the Invention
The present invention relates to a flat motor that is so called an axial gap type flat motor in which a driving magnet faces toward a driving coil in the plane, particularly, relates to a spindle motor for flexible disc drive (FDD) that is essential to be thinner.
2. Description of the Related Art
A so-called axial gap type flat motor in which a driving magnet and a driving coil is constituted to be faced toward with each other in the plane is utilized for various equipment such as an FDD.
With referring to FIGS. 9(a), 9(b) and 10, a conventional flat motor for FDD is explained.
FIGS. 9(a) and 9(b) are cross-sectional views of a flat motor according to the prior art.
FIG. 10 is an enlarged fragmentary cross-sectional view of the flat motor shown in FIG. 9(a).
In FIGS. 9(a) and 10, a flat motor is composed of a rotor 100 that is formed by a soft magnetic material and a stator 50. The rotor 100 is further composed of a rotor yoke 20 and a ring-shaped driving magnet 30 that is adhered on the rotor yoke 20. A lower surface of the ring-shaped driving magnet 30 is magnetized with multipolar driving magnetic poles.
On the other hand, the stator 50 that is disposed under the rotor 100 is constituted by a plurality of ring-shaped driving coils 40, which is adhered in a ring shape on a plane stator yoke 60 that is to be a part of magnetic circuit and formed by a soft magnetic material.
Further, a shaft 80 to be an axis of revolution is installed in the rotor 100. The shaft 80 is held in the center of the stator 50 by way of a bearing 70 to rotate freely.
A slight gap xe2x80x9cd1xe2x80x9d is provided between the ring-shaped driving coil 40 and the ring-shaped driving magnet 30 to eliminate interference between them while the rotor 100 is rotating.
The rotor 100 rotates by rotational driving force that is generated by interaction between a magnetic field caused by the multipolar driving magnetic poles and rotational magnetic field caused by applying predetermined electric current to the ring-shaped driving coil 40.
Recently, a spindle motor for FDD is strongly required to be smaller in size and lighter in weight. Consequently, a rare-earth magnet having strong suction power is utilized for a ring-shaped driving magnet in order to increase operational efficiency while a thickness of motor is made thinner.
The stator yoke 60 is strongly drawn by the ring-shaped driving magnet 30 because of strong suction power, and resulted in deforming its shape such that the stator yoke 60 approaches to the ring-shaped driving magnet 30. Consequently, the gap xe2x80x9cd1xe2x80x9d between the ring-shaped driving magnet 30 and the ring-shaped driving coil 40 is narrowing in accordance with being toward the outer circumference of the stator yoke 60.
The Japanese Patent Application Laid-open Publications No. 6-11163/1994 discloses a structure of the stator yoke 60. As shown in FIG. 9(b), the structure is such that the stator yoke 60 is bent toward the opposite direction to the rotor 100 in accordance with being away from the axis of revolution when assembled.
The shape of the stator yoke 60 is, as shown in FIG. 9(a), formed like a saucer to maintain the gape xe2x80x9cd1xe2x80x9d between the ring-shaped driving magnet 30 and the ring-shaped driving coil 40 to be proper distance when the stator 50 is assembled with the rotor 100.
Generally, rotational torque increases in accordance with narrowing a gap between a driving magnet and a driving coil because magnetic flux generated by the driving magnet effectively links with the driving coil. Consequently, a motor is led to have higher efficiency, and the motor can be made thinner. However, in the case of the aforementioned stator yoke formed in a shape of saucer, deformation of the stator yoke caused by the strong suction power that acts between the driving magnet and the stator yoke is not uniform. Consequently, there existed a wider gap and a narrower gap between the driving coil and the stator yoke in the radial direction.
More specifically, the stator yoke approaches to the driving coil and the gap between them becomes narrower in accordance with the direction from the driving coil towards a spindle shaft. On the contrary, the stator yoke is deformed such that the gap between the driving magnet and the driving coil gradually increases in accordance with the direction towards the outer circumference.
Accordingly, there existed a restriction for the gap xe2x80x9cd1xe2x80x9d to be made narrower and resulted in a problem such that thinning the motor further more is hardly conducted.
Further, there existed another problem such that motor characteristics change in accordance with elapsed time and resulted in less reliability. Because, the stator yoke is always strained by the strong suction power and deformed gradually, and further, a creep phenomenon that changes drastically by external thermal load occurs in the stator yoke. By the creep phenomenon, the gap between the driving magnet and the driving coil is narrowed and resulted in a further problem. In the worst case, the driving magnet and the driving coil interferes with each other and are possibly damaged while revolving.
Accordingly, in consideration of the above-mentioned problems of the prior art, an object of the present invention is to provide a flat motor, which can be thinned more and has less change of characteristic by time elapsed and is high in reliability.
In order to achieve the above object, the present invention provides, according to an aspect thereof, a flat motor comprising a rotor composed of an axis and a driving magnet in a ring shape having multipolar driving magnetic poles; and a stator yoke composed of a plane section for mounting a bearing for holding the rotor to rotate freely with centering the axis and another plane section for mounting a plurality of driving coils disposed in a concentric circle with facing towards the driving magnet, wherein the other plane section is disposed in conjugating with the outer side of the plane section, the flat motor is further characterized in that the plane section is formed flat in a circular shape with centering the axis, and the other plane section is a part of surface of approximately a circular cone shape having a vertex on the axis and is in a shape of inclining towards a direction away from the rotor in accordance with being far away from the axis and formed with a pleat by bending the stator yoke in a conjugating section between the plane section and the other plane section.
Other object and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.